New Cold Subdwarf Discoveries from Backyard Worlds and a Metallicity Classification System for T Subdwarfs
Abstract We report the results of a spectroscopic survey of candidate T subdwarfs identified by the Backyard Worlds: Planet 9 program. Near-infrared spectra of 31 sources with red J − W2 colors and large J -band reduced proper motions show varying signatures of subsolar metallicity, including strong collision-induced H 2 absorption, obscured methane and water features, and weak K i absorption. These metallicity signatures are supported by spectral model fits and 3D velocities, indicating thick disk and halo population membership for several sources. We identify three new metal-poor T subdwarfs ([M/H] ≲ –0.5), CWISE J062316.19+071505.6, WISEA J152443.14−262001.8, and CWISE J211250.11-052925.2; and 19 new “mild” subdwarfs with modest metal deficiency ([M/H] ≲ −0.25). We also identify three metal-rich brown dwarfs with thick disk kinematics. We provide kinematic evidence that the extreme L subdwarf 2MASS J053253.46+824646.5 and the mild T subdwarf CWISE J113010.07+313944.7 may be part of the Thamnos population, while the T subdwarf CWISE J155349.96+693355.2 may be part of the Helmi stream. We define a metallicity classification system for T dwarfs that adds mild subdwarfs (d/sdT), subdwarfs (sdT), and extreme subdwarfs (esdT) to the existing dwarf sequence. We also define a metallicity spectral index that correlates with metallicities inferred from spectral model fits and iron abundances from stellar primaries of benchmark T dwarf companions. This expansion of the T dwarf classification system supports investigations of ancient, metal-poor brown dwarfs now being uncovered in deep imaging and spectroscopic surveys.
Authors
- Adam C. Schneider (ORCID: https://orcid.org/0000-0002-6294-5937)
- Jean Marc Gantier (ORCID: https://orcid.org/0000-0002-1044-1112)
- Jacqueline K. Faherty (ORCID: https://orcid.org/0000-0001-6251-0573)
- Paul Beaulieu
- Jim Walla
- Martin Kabatnik (ORCID: https://orcid.org/0000-0003-4905-1370)
- Dan Caselden (ORCID: https://orcid.org/0000-0001-7896-5791)
- William Pendrill
- Adam J. Burgasser (ORCID: https://orcid.org/0000-0002-6523-9536)
- Ken Hinckley (ORCID: https://orcid.org/0000-0002-4733-4927)
- Frank Kiwy (ORCID: https://orcid.org/0000-0001-8662-1622)
- J. Davy Kirkpatrick (ORCID: https://orcid.org/0000-0003-4269-260X)
- Thomas P. Bickle (ORCID: https://orcid.org/0000-0003-2235-761X)
- Jonathan Gagné (ORCID: https://orcid.org/0000-0002-2592-9612)
- Leslie K. Hamlet (ORCID: https://orcid.org/0000-0002-7389-2092)
- Zbigniew Wędracki
- Arttu Sainio (ORCID: https://orcid.org/0000-0003-4864-5484)
- Christopher A. Theissen (ORCID: https://orcid.org/0000-0002-9807-5435)
- Hunter Brooks (ORCID: https://orcid.org/0000-0002-5253-0383)
- Austin Rothermich (ORCID: https://orcid.org/0000-0003-4083-9962)
- Chih-Chun Hsu (ORCID: https://orcid.org/0000-0002-5370-7494)
- Michaël Marsset (ORCID: https://orcid.org/0000-0001-8617-2425)
- Nikolaj Stevnbak Andersen (ORCID: https://orcid.org/0000-0003-4714-3829)
- Aaron Meisner (ORCID: https://orcid.org/0000-0002-1125-7384)
- Emma Softich (ORCID: https://orcid.org/0000-0002-1420-1837)
- Melina Thévenot (ORCID: https://orcid.org/0000-0001-5284-9231)
- Léopold Gramaize (ORCID: https://orcid.org/0000-0002-8960-4964)
- Étienne Artigau (ORCID: https://orcid.org/0000-0003-3506-5667)
- Christian Aganze (ORCID: https://orcid.org/0000-0003-2094-9128)
- J. Schümann (ORCID: https://orcid.org/0000-0002-7587-7195)
- Guillaume Colin (ORCID: https://orcid.org/0000-0002-7630-1243)
- Marc J. Kuchner (ORCID: https://orcid.org/0000-0002-2387-5489)
- Preethi R. Karpoor (ORCID: https://orcid.org/0000-0002-1480-9041)
- Roman Gerasimov (ORCID: https://orcid.org/0000-0003-0398-639X)
- David W. Martin
- Diego H. Massat
Publication Details
- Journal
- The Astrophysical Journal
- Published
- 2025-03-20
- DOI
- https://doi.org/10.3847/1538-4357/adb39f
- Citations
- 13
- Primary Topic
- Geological and Geochemical Analysis
- Type
- article
- Field-Weighted Citation Impact
- 24.26